This invention relates to an amplifier circuit useful, among other things, in wireless devices such as cellular telephones to amplify radio frequency signals.
Amplifier circuits are used, among other things, to amplify a radio frequency (RF) signal within a wireless device such as a cellular telephone or personal data assistant. It is known in the art to construct such an amplifier circuit from a number of heterojunction bipolar transistors (HBTs) and also known that HBTs suffer from thermal runaway. See U.S. Pat. Nos. 5,629,648, 5,321,279 and 5,608,353 incorporated herein by this reference. In the ""648 and ""353 patents, the amplifier circuit includes a number of amplifiers each including an HBT with an emitter coupled to ground, a base, a collector, a base resistor, and a base capacitance in the form of a special segmented capacitor for each HBT to reduce the chances of thermal runaway.
Because the emitters of each HBT are coupled directly to ground, there is no emitter ballasting even though that technique is known to also reduce thermal runaways. See the ""648 patent, col. 1, lines 43-50.
In the ""279 patent, ballast impedances are added to the base fingers of an HBT but again, emitter ballasting is not provided for. Other relevant art includes U.S. Pat. No. 5,760,457 and the articles xe2x80x9cThe Use of Base Ballasting to Prevent the Collapse of Current Gain in ALGaAs/GaAs HBTsxe2x80x9d, W. Lui, et al., IEEE Transactions on Electronic Devices, Vol. 43, No. 2, February 1996; xe2x80x9cHandbook of III-IV HBTsxe2x80x9d, W. Lui, John Wiley and Sons, 1998; and xe2x80x9cHorowitz and Hill: The Art of Electronicsxe2x80x9d, Paul Horowitz, Winfield Hill, Cambridge University Press, NY, N.Y. 1980, also included herein by this reference.
Employing both base and emitter ballasting for the HBTs in accordance with the prior art typically requires a large area circuit resulting in high fabrication costs.
It is therefore an object of this invention to provide an amplifier with base ballasting and also, preferably, emitter ballasting to fully address thermal runaway.
It is a further object of this invention to provide such an amplifier at a lower cost.
It is a further object of this invention to provide such an amplifier which does not require a large circuit area.
This invention results from the realization that by forming amplifier cells with emitter fingers, base fingers, and collector fingers, and by splitting the base ballasting amongst different base fingers, then base and also, if desired, emitter ballasting can be provided to prevent thermal runaway and, at the same time, the layout area is optimized.
This invention features an amplifier circuit comprising a bias input node, an RF input node, an RF output node, and a plurality of amplifier cells. The cells each preferably include a plurality of discrete emitter contacts of a first conductivity type, a plurality of discrete base contacts of a second conductivity type and grouped in two or more groups, at least one collector contact of the first conductivity type connected to the RF output node, and a base capacitor for each group having two electrodes: an input electrode coupled to said RF input node and an output electrode coupled to a group of discrete base contacts. There is a base resistor for each group having an input coupled to the bias input node and an output coupled to a group of discrete base contacts. An emitter resistor may be coupled to each discrete emitter contact to provide more effective ballasting and thermal stability than with a paralleled arrangement of HBT transistors. Typically, the number of base contacts in each group is the same.
One amplifier circuit in accordance with this invention features a bias input node, a signal input node, a signal output node, and a plurality of amplifier cells each including a plurality of discrete emitter contacts of a first conductivity type, a plurality of discrete base contacts of a second conductivity type and grouped in two or more groups, a base capacitor for each group, each base capacitor having two electrodes, an input electrode coupled to said signal input node, and an output electrode coupled to a group of discrete base contacts, and a base resistor for each group, each base resistor having an input coupled to the bias input node and an output coupled to a group of discrete base contacts. Each cell may also include at least one collector contact coupled to the signal output node.
Another amplifier circuit in accordance with this invention features a bias input node, a signal input node, a signal output node, and a plurality of amplifier cells each including a plurality of discrete emitter contacts of a first conductivity type, a plurality of discrete base contacts of a second conductivity type and grouped in two or more groups and coupled to the signal input node, at least one collector contact of the first conductivity type coupled to the signal output node, and a base resistor for each group, each base resistor having an input coupled to the bias input node and an output coupled to a group of discrete base contacts. Each cell may also include a base capacitor for each group of base contacts.
An amplifier cell in accordance with this invention typically has a plurality of discrete emitter contacts of a first conductivity type, a plurality of discrete base contacts of a second conductivity type and grouped in two or more groups, at least one collector contact of the first conductivity type, a base capacitor for each group, each base capacitor having two electrodes, an input electrode coupled to a signal input node and an output electrode coupled to a group of discrete base contacts, a base resistor for each group, each base resistor having an input coupled to a bias input node and an output coupled to a group of discrete base contacts, and an emitter resistor coupled to each discrete emitter contact.
One amplifier circuit fabrication method in accordance with this invention includes the steps of providing a bias input node, providing an RF input node, providing an RF output node, and forming a plurality of amplifier cells to each include: a plurality of discrete emitter contacts of a first conductivity type, a plurality of discrete base contacts of a second conductivity type grouped in two or more groups, at least one collector contact of the first conductivity type connected to the RF output node, a base capacitor for each group, each base capacitor having two electrodes, an input electrode coupled to said RF input node, and an output electrode coupled to a group of discrete base contacts, a base resistor for each group, each base resistor having an input coupled to the bias input node and an output coupled to a group of discrete base contacts, and an emitter resistor coupled to each discrete emitter contact to provide more effective ballasting and thermal stability than with a paralleled arrangement of HBT transistors.
Another amplifier circuit fabrication method comprises: providing a bias input node, providing a signal input node, providing a signal output node, and forming a plurality of amplifier cells to each include a plurality of discrete emitter contacts of a first conductivity type, a plurality of discrete base contacts of a second conductivity type grouped in two or more groups, a base capacitor for each group, each base capacitor having two electrodes, an input electrode coupled to said signal input node, and an output electrode coupled to a group of discrete base contacts, and a base resistor for each group, each base resistor having an input coupled to the bias input node and an output coupled to a group of discrete base contacts. This method may further include forming a collector contact coupled to the signal output node.
Still another amplifier circuit fabrication method in accordance with this invention features providing a bias input node, providing a signal input node, providing a signal output node, and forming a plurality of amplifier cells to each include: a plurality of discrete emitter contacts of a first conductivity type, a plurality of discrete base contacts of a second conductivity type grouped in two or more groups and coupled to the signal input node, at least one collector contact of the first conductivity type coupled to the signal output node, and a base resistor for each group, each base resistor having an input coupled to the bias input node and an output coupled to a group of discrete base contacts. This method may further include the step of providing a base capacitor for each base contact group.
An amplifier cell fabrication method in accordance with this invention features the steps of forming a plurality of discrete emitter contacts of a first conductivity type, forming a plurality of discrete base contacts of a second conductivity type grouped in two or more groups, forming at least one collector contact of the first conductivity type, forming a base capacitor for each group, each base capacitor having two electrodes, an input electrode coupled to a signal input node and an output electrode coupled to a group of discrete base contacts, forming a base resistor for each group, each base resistor having an input coupled to a bias input node and an output coupled to a group of discrete base contacts, and forming an emitter resistor coupled to each discrete emitter contact.